The prize was a poster ... a poster of which only two others exist in the world—one belongs to my sister, and one hangs in my office (below). It's the rejected cover of Babymouse: Monster Mash:

(Note that it wasn't rejected for the reason you might think—scariness and general zombiehood—but rather because people feared that Babymouse wouldn't be immediately recognizable as Babymouse since she's wearing a suit. And, to be fair, she's not smiling as she is on every single other Babymouse cover.)